1. Technical Field of the Invention
The present invention relates generally to a air-fuel ratio sensor monitor designed to monitor response characteristics of an air-fuel ratio sensor for internal combustion engines and a failure in operation of the air-fuel ratio sensor, an air-fuel ratio detector designed to detect an air-fuel ratio of a mixture to the engine using an air-fuel ratio sensor, and an air-fuel ratio control designed to control an air-fuel ratio of a mixture to the engine using an air-fuel ratio sensor.
2. Background Art
Air-fuel ratio detecting devices have already been put into practical use which have an air-fuel ratio sensor (e.g., an exhaust gas oxygen sensor) installed in an exhaust pipe of an internal combustion engine which produces an indication of an instantaneous air-fuel ratio that is being used by the engine. In recent years, as such a type of air-fuel ratio sensor, linear air-fuel ratio sensors have been employed which produce an output changing linearly with the instantaneous air-fuel ratio. Air-fuel ratio control systems using such an air-fuel ratio detecting device work to bring an air-fuel ratio, as measured by the air-fuel ratio sensor, into agreement with a target one under feedback control, thereby improving exhaust emissions of the engine.
It is important for the air-fuel ratio feedback control to ensure the stability of operation of the air-fuel ratio sensor at all times. For instance, Japanese Patent First Publication No. 4-237851 teaches diagnosing the deterioration of the air-fuel ratio sensor using a sensor response rate when an air-fuel ratio feedback gain is changed around the stoichiometric air-fuel ratio. U.S. Pat. No. 5,964,208, assigned to the same assignee as that of this application, discloses an air-fuel ratio control system which determines a rate of change in air-fuel ratio, as detected by an air-fuel ratio sensor, and a rate of change in air-fuel ratio correction factor and compares them to diagnose the sensor.
The most common type of air-fuel ratio sensor is an oxygen sensor made up of a zirconia solid electrolyte body with two electrodes affixed thereto. The oxygen sensor works to ionize oxygen molecules contained in exhaust gas of the engine and measure the amount of oxygen ions moving between the electrodes as representing the concentration of oxygen in the exhaust gas which depends upon an instantaneous air-fuel ratio of a mixture to the engine. However, such a type of oxygen sensor may have a difference between response rates when the air-fuel ratio changes to a rich side and to a lean side due to original reactive errors or aging of the sensor. This results in a difficulty in diagnosing the sensor accurately if the response rate of the sensor drops undesirably only at either one of rich and lean mixtures.
U.S. Pat. No. 5,119,629 discloses an air-fuel ratio feedback controls system using the above type of air-fuel ratio sensor in order to improve emission control efficiency of a catalytic converter. Such a feedback control system, however, has a problem that the accuracy of determining the air-fuel ratio decreases due to the above described response rate difference of the air-fuel ratio sensor between rich and lean mixtures.
Japanese Patent First Publication No. 2-67443 teaches an air-fuel ratio control system which has a linear air-fuel (A/F) ratio sensor installed upstream of a three-way catalytic converter and a λO2 sensor installed downstream of the converter and monitors an output of the λO2 sensor to correct controlled variables of the linear A/F ratio sensor and air-fuel ratio correction factors. This type of control system also encounters the same problem as described above, thus resulting in a variation in speed at which the air-fuel ratio converges on the stoichiometric air-fuel ratio.